1. Field of the Invention
This invention relates in general to the field of wafer processing and to rapid, uniform heating and cooling of semiconductor wafers during integrated circuit manufacturing in particular.
2. Discussion of Related Art
Temperature uniformity across a wafer surface is an important factor in many integrated circuit fabrication steps since rate processes used in fabrication are generally temperature dependent. In particular processes, thermal non-uniformity can adversely effect a circuit element""s critical dimension size by enlarging it relative to a design goal. Thus, as trends toward smaller integrated circuit critical dimensions continue, temperature uniformity requirements for wafer thermal management systems will become increasingly stringent.
For example, as compared to earlier standards, current temperature uniformity requirements of 0.1xc2x0 C. across a several hundred millimeters diameter wafer represent a 50% increase in temperature uniformity. Wafer heating and cooling units must be able to achieve such temperature uniformity figures of merit over a wide range of process operating temperatures, typically from about 20xc2x0 C. to several hundred degrees Celsius, and in a manner consistent from wafer-to-wafer. Rapid thermal management over the operating temperature range is also desirable for economy of process.
In the wafer processing art, stringent requirements for efficient, rapid, controllable and uniform thermal management over a wide temperature range are not fully met in an integrated device. In examples of closely related art, separate devices carry out heating and cooling functions, but as a mere combination of separate heating and cooling units positioned in opposition to each other. In these devices, a wafer is interposed between the heating and cooling elements and is translated between the elements.
An example of such a device is shown in FIG. 1. In FIG. 1, semiconductor wafer 110, heating surface 120, and cooling surface 130 are shown. A hot thermal source and a cold thermal sink interacting with the wafer are not integrated. Rather, they are physically separated or otherwise thermally isolated from each other. To heat or cool the semiconductor wafer, the wafer is mechanically translated in a space between the heating and cooling surfaces according to the desired function. This is inefficient for the manufacturing process flow and clean-room floor space utilization, as compared to an integrated thermal management device that performs both heating and cooling functions.
Thus, due to the limitations of related art, there is a need for methods of efficient, rapid, controllable and uniform thermal management over a wide temperature range. Further, there is a need for an apparatus that provides the desired high performance during both steady state and transient conditions and that is very suitable for easy use within a manufacturing setting.
This invention is a method of substrate thermal management for efficient, rapid, controllable and uniform thermal management over a wide temperature range.
One object of the thermal management method is to integrate a thermal source, thermal sink and a thermal diffuser. According to the invention, a thermal diffuser is positioned stationary relative to the wafer surface and coupled to a thermal source and a thermal sink, which are also stationary relative to the wafer surface. The thermal sink comprises a heat-carrying media with a controllable temperature. The wafer is heated from a first processing temperature to a second processing temperature during a heating time interval and then cooled to the first processing temperature from the second processing temperature during a cooling time interval. During this process, the wafer is constantly held in a fixed position.
Another object of the thermal management method is to minimize thermal non-uniformity across a wafer surface by diffusing heat from the heating elements through a thermal diffuser made of thermally conductive material to a heating surface facing the wafer, thereby smoothing temperature gradients of the thermal source.
Yet another object of the thermal management method is to minimize thermal non-uniformity across a wafer surface by non-uniform control of zones in a thermal source. According to the invention, a thermal source includes a plurality of zones adapted to provide differing amounts of heat and a controllable temperature field over a surface proximal to a thermal diffuser. The thermal diffuser is diffuses thermal gradients and is positioned stationary relative to the wafer surface and coupled to a thermal source and a thermal sink, which are also stationary relative to the wafer surface. The wafer is cooled by a thermal sink comprises a heat-carrying media with a controllable temperature.
Yet another object of the thermal management method is to minimize thermal non-uniformity across a wafer surface by flowing a thermal sink material of controllable temperature through passages of a thermal reservoir in a non-uniform manner. According to the invention, such a non-uniform flow may establish a controllable, distributed and substantially uniform thermal sink temperature field.